Apparatus
Flasks, beakers, filter funnels, pipettes, filter crucibles, filter papers, oven, muffle furnace, chemical
balance, desiccator.
Applications
Extensive numbers of inorganic ions are determined with excellent precision and accuracy; widely used
in routine assays of metallurgical and mineralogical samples. Relative precision 0.1–1%.
Disadvantages
Requires careful and time-consuming procedures with scrupulously clean apparatus and very accurate
weighings. Coprecipitation cannot always be avoided.
Gravimetry includes any analytical method in which the ultimate measurement is by weight. The
simplest form may merely be the drying or heating of a sample in order to determine its volatile and
non-volatile components, or possibly the sample might be distilled and the residue and fractions of
distillate weighed. Metals may be deposited electrolytically and weighed (Chapter 6). Of far greater
scope and importance is the controlled precipitation of an analyte from solution, followed by the
weighing of the precipitate. Subsequent attention will be restricted to these precipitation methods.
To be of gravimetric value, a precipitation process must fulfil certain conditions. The precipitate must
be formed quantitatively and within a reasonable time. Its solubility should be low enough for a
quantitative separation to be made. It must be readily filterable and, if possible, have a known and
stable stoichiometric composition when dried so that its weight can be related to the amount of analyte
present. Failing this, it must be possible to convert the precipitate to a stoichiometric weighable form
(usually by ignition). In both cases the weighed form should be non-hygroscopic.
Precipitation Reactions
If it is remembered that a chemical reaction can be displaced by changing the state of the products
(Chapter 3) then, provided the solubility product of a precipitate is small, quantitative reaction can be
obtained. Practically, the precipitate may be formed directly on the addition of a reagent or on the
subsequent adjustment of solution conditions, e.g. pH. Precipitates may be of different chemical types,
e.g. salts, complexes, hydroxides, hydrous oxides, and precipitating agents are conveniently divided
into inorganic (Table 5.14) and organic (Table 5.15). The major areas of applications usually employ
inorganic precipitants for inorganic analytes.